feat: add rtk as custom Nix derivation

Build rtk from source (not yet in nixpkgs) using rustPlatform.buildRustPackage.
Custom derivations extracted to pkgs/ for single responsibility.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages.nix

@@ -3,6 +3,7 @@
 let
   atomicptr = import (fetchTarball
     "https://github.com/atomicptr/nix/archive/refs/heads/master.tar.gz") { };
+  rtk = pkgs.callPackage ./pkgs/rtk.nix { };
 in {
   nixpkgs.config.allowUnfreePredicate = pkg:
     builtins.elem (lib.getName pkg) [
@@ -127,7 +128,7 @@ in {
     playerctl
     numlockx
 
-    nixfmt-classic
+    nixfmt
     nodejs_22
     fnm
     bun
@@ -162,6 +163,8 @@ in {
     shfmt
     toilet
 
+    rtk
+
     # AI
     ollama
     whisper-cpp # STT local